1. Field of the Invention
The present invention relates to photolithographic techniques used in thin-film head (TFH) processing. More particularly, the present invention relates to tri-layer anti-reflective coatings that minimize pattern distortion due to reflections from features having severely angled topologies.
2. Description of Related Art
Thin-film magnetic recording heads have gained wide acceptance in the data storage industry. A thin-film recording head has small, precisely formed structures that are formed using several thin-film deposition approaches. Manufacturers seek to form the geometry of the recording heads with high precision, and to thereby achieve geometries that provide magnetic field patterns suitable for writing narrow tracks of recorded data.
Typically, the lithographic technique deposits alternating layers of conductive and insulating materials onto a substrate by evaporation, sputtering, plating, or other deposition technique that provides precise control of the deposition thickness. Chemical etching, reactive ion etching (RIE), or other mechanisms shape and form the deposited layers into a pole-tip assembly having the desired geometry. Thus, the head structure is a multi-layer lithographically fabricated device.
Although existing lithographic techniques work sufficiently well to provide head structures, e.g., pole-tip assemblies, with feature sizes suitable for current data storage capacity, these lithographic techniques are limited as to the small feature sizes that they can produce.
Thin film heads require sharply defined photoresist patterns because these patterns are used to define the locations (and density) of structures formed. In a thin film process, a thin layer of photoresist may be applied to the surface of a wafer. The wafer is heated in a process called soft baking, wherein partial evaporation of photoresist solvents evaporate. A mask is then aligned over the wafer, wherein the mask allows light to pass through its clear areas and be blocked by opaque areas during a light exposure step. However, during the exposure step, light reflects from the surface of an underlying substrate (or neighboring features) over which the photoresist is formed. For example, materials that are used to form the thin film head structure are highly reflective, e.g., copper, tantalum and alloys of nickel, iron and cobalt. Reflections from the surface of the substrate underlying the photoresist also causes deleterious effects that limit the resolution of photolithographic photoresist patterning.
These deleterious effects are caused during the development step by light passing through the photoresist at least twice, rather than only once. This occurs because light is reflected from a surface of the underlying substrate and components (or features) and passes back through the photoresist layer a second time. The chemical structure of the photoresist changes differently when light passes through the photoresist more than once. A portion of the light, already reflected from the surface of the underlying substrate can also reflect again from the surface of the photoresist, passing back through the photoresist yet again. In fact, standing light waves can result in the photoresist from superpositioning of incident and reflected light rays. These reflections result in process latitude and control problems.
The reflection of the light reduces the sharpness of the resulting photoresist pattern. A portion of the light reflected obliquely from the surface of the underlying substrate can also be again reflected obliquely from the surface of the photoresist. As a result of such angular reflections, the light can travel well outside those photoresist regions underlying the transmissive portions of the photolithographic mask. This potentially causes photoresist exposure well outside those photoresist regions underlying transmissive portions of the photolithographic mask. This problem results in a less sharply defined photoresist pattern that limits the density of structures formed on the integrated circuit.
The problem occurring from such reflections during the exposure step have been addressed by applying a bottom anti-reflective coating (BARC). The BARC layer minimizes reflected light during the photoexposure step thereby resulting in more faithfully reproduced trench walls. However, in most anti-reflective coatings, the anti-reflective layers need to be fine-tuned for greater process latitude. Thus, the thickness of the layers of the anti-reflective coating must be precisely controlled to obtain proper absorption of the reflected light in a particular application. Also, some BARCs are spun-on to wafers. When these spun-on coatings are applied over features with significant topography, the thickness of the coating varies. This can compromise their effectiveness as a BARC and make subsequent processing (removal) difficult or impossible.
In addition, after photoresist exposure, a BARC must be cleared from the developed-away regions of the photoresist. This may be performed using RIE, wet chemistry, sputter etch, ion milling, etc. Thinner, more uniformly distributed BARCs permit quick removal within minimum “over etch.” This is important since these processes may have the undesired side effect of consuming some of the critical dimension (CD) budget because they (depending on materials choices and chemistry) consume photoresist or re-deposit non-volatile BARC-byproducts on the photoresist sidewalls.
Some bi-layer ARCs have been shown to dramatically reduce reflections in structures with low angle topographies (less than 30 degree) because of the reflection off the low angle features and absorption characteristics. The bi-layer ARC has been extremely useful in the recessed P3 (pole piece layer) process. However, the physics of a bi-layer stack is limited in its ability to achieve near zero reflectance at one of two highly different angles with respect to the wafer surface. For processes such as P3, the second angle is small enough such that reflectance can be minimized to near 5% for each angle (0 and 30 degrees). Nevertheless, for processes with more severe topography, for example those involving a dual layer coil, for example, the required angles to eliminate reflection become more like 0 and 60 degrees. In a bi-layer ARC design, it is physically impossible to achieve near zero reflectance at both of these angles.
It can be seen that there is a need to create an effective anti-reflective layer for wider topographies.
It can also be seen that there is a need to provide a thinner anti-reflective layer with wider process latitude.